Semiconductor memory devices are typically manufactured with circuits, such as redundancy circuits, to address defects that can arise from uncontrollable manufacturing process variations, and the like. Unfortunately, semiconductor memory devices are still vulnerable to errors that may arise out of the operating environment and conditions, such as “soft” errors.
“Soft” errors are random errors that can appear in a memory device that are not related to any defect. Soft errors are most often attributed, either directly or indirectly, to sub-atomic particles. More particularly, many soft errors are believed to result from alpha particles traveling through a semiconductor substrate and generating electron hole pairs. Alpha particles may be generated by the decay of elements (e.g., uranium, thorium) or cosmic ray events, for example.
In a soft-error event, generated electron hole pairs can cause a change in state of the data stored within a memory device. For example, in the case of a dynamic random access memory (DRAM) type cell, generated electron hole pairs may result in loss of charge from a storage capacitor. In the case of a latch-based static RAM (SRAM) type cell, the cell may “flip”, resulting in the opposite logic value being stored. Soft errors in semiconductor devices are often described in terms of a soft-error rate (SER). An SER can be an indication of an integrated circuit's degree of susceptibility to soft-error events.
One way of reducing the SER in DRAM devices can be to increase the capacitance of a storage cell. However, this approach has disadvantages that may include increased current consumption and/or larger memory cell sizes.
Other approaches to reducing the SER of semiconductor devices include forming a thick polymer coating and/or a phosphosilicate glass (PSG) type of layer on top of the device. Such a coating may act to absorb some soft-error particles before they can impact the memory cells. Also, materials used to package devices may be manufactured to have low levels of radioactive impurities so as to reduce the possible sources of unwanted particles.
Unfortunately, as semiconductor memory devices continue to shrink in size, the number of electron hole pairs necessary to induce a soft error can be correspondingly smaller. Thus, it remains a continuing goal to reduce the soft error rate (SER) of semiconductor memory devices.